A foil blowing apparatus can comprise an extruder having an annular-gap nozzle from which a primary tube of the thermoplastic synthetic resin is extruded, this tube having a hose-like character and being relatively pliable. Upon cooling of the extruded plastic tube, integrity is imparted thereto and the plastic tube is flattened in the cooled state so that it remains in a flattened condition until it is supplied to a heating stretch where it is heated to a plastically deformable state, hereinafter referred to as the blowing temperature. At the blowing temperature, the primary tube is expanded into a balloon or bubble to form a longitudinally stretched and circumferentially stretched product which can be referred to as a secondary tube with thinning of the wall to the thickness of a foil. Downstream of the blowing station, a flattening unit is provided to flatten the foil balloon or bubble and the web of blown foil can then be wound up or coiled.
Generally speaking, at the end of the primary tube beneath the annular-gap nozzle and ahead of the heating stretch and at the outlet side of the flattening unit ahead of the foil coiling unit, drawing roller pairs are provided which can apply tension to the tubes to draw the primary tube downwardly until it reaches the first roller pair at the end of the primary tube and at which the primary tube is flattened.
From the primary drawing roller pair, the flattened primary tube can pass upwardly to a second drawing roller pair at the inlet to the heating stretch and the secondary tube is provided below the foil-blowing station to draw the foil tube downwardly with flattening of the foil.
While the first primary tube section is generally vertical and downwardly oriented, it can be inclined downwardly as well.
The next section of the primary tube, already flattened, usually passes at an inclination upwardly to the second drawing roller pair.
An apparatus of this type can be provided in a variety of configurations. The primary tube can have a single layer wall or two or more layers forming the wall thickness. The apparatus can thus be used to produce simple foils or a composite or multilayer or laminate foils. The biaxial stretching is effected by a superimposition of axial stretching in the longitudinal direction of the primary tube with a transverse or circumferential stretching in the blowing of the foil bubble. The axial stretching can be effected by the action of the drawing roller pairs and the stretching resulting from the blowing can be orthogonal to the axial stretching.
The cooling of the primary tube is effected upon emergence of the actuated tube from the annular gap and utilizing air or water.
Difficulties have been encountered with such apparatus with respect to the uniformity and reproducibility of the product and with respect to start up of the apparatus. Indeed, numerous manipulations of the equipment can be required for start up which may need expert intervention. In fact, the primary tube must not only be threaded through the first pair of drawing rollers but the tubes must be fed through the entire system by hand for start up all the way through to the coiling unit. The apparatus must be slowly brought to the blowing operation by the gradual introduction of blowing air into the heated stretch of the primary tube. The start-up process is thus time-consuming until the desired stretching ratio is reached.